The invention relates to methods and systems for detecting selected isotopes of all noble gases. More particularly, the invention is directed to methods and systems for performing atomic counting based on the atom trap trace analysis method (“ATTA”) to measure ratios of 81Kr/Kr and 85Kr/Kr in environmental samples.
Krypton permeates through the atmosphere at a concentration of about one part per million. There are six stable Kr isotopes, and two rare, long-lived isotopes: 81Kr(t1/2=2.29×105 yr, isotopic abundance 81Kr/Kr=6×10−13) and 85Kr (t1/2=10 yr, 85Kr/Kr−10−11). Upon the discovery of 81Kr in the atmosphere, it has been proposed that 81Kr is an ideal tracer isotope for dating water and ice in the age range of 105-106 years, a range beyond the reach of 14C-dating. 81Kr is mainly produced in the upper atmosphere by cosmic-ray induced spallation and neutron activation of stable Kr. Due to its long residence time, 81Kr is expected to be distributed uniformly throughout the atmosphere. Subsurface sources and sinks for 81Kr other than radioactive decay are most likely negligible. Human activities involving nuclear fission have a negligible effect on the 81Kr concentration because the stable 81Br shields 81Kr from the decay of the neutron-rich fission products. All of these favorable conditions combine to support the case for 81Kr-dating. The other long-lived krypton isotope, 85Kr, has a completely different production source. It is a fission product of 235U and 239Pu, and is released into the atmosphere primarily by nuclear fuel reprocessing activities. 85Kr can be used as a tracer to study air and ocean currents, determine residence time of young groundwater in shallow aquifers and monitor nuclear fuel processing activities.
For 85Kr analysis, low level decay counting (LLC) is performed routinely in a few specialized laboratories around the world. LLC was also the first method used to detect 81Kr and to determine its abundance in the atmosphere, but it is too inefficient for practical 81Kr-dating because only a fraction 3×10−8 of 81Kr atoms in a sample decays in a 100-hour measurement. In general, counting atoms is preferable to counting decays for analyses of long-lived isotopes because of the enhanced efficiency, and because of the immunity to other decay backgrounds from both the sample and the surroundings. An accelerator mass spectrometry (AMS) method for counting 81Kr ions has been developed, and has been used to perform 81Kr-dating of four groundwater samples from the Great Artesian Basin of Australia. However, due to the complexity of this technique, which required the use of a high energy (4 GeV) cyclotron to produce fully stripped 81Kr ions, and the large sample size required (16 tons of water), the AMS effort on 81Kr-dating was halted following these proof-of-principle measurements.
Atom Trap Trace Analysis (ATTA) is another type of atom-counting method capable of detecting both 81Kr and 85Kr in environmental samples. In ATTA, an atom of a particular isotope is selectively captured by resonant laser light in a magneto-optical trap (MOT) and detected by observing its fluorescence. Following the first demonstration of ATTA, both the reliability and counting efficiency of the ATTA instrument have been improved. An earlier version, ATTA-2, had a counting efficiency of 1×10−4 and, for each 81Kr/Kr analysis, needed a sample of 50 μL STP of Kr gas extracted from approximately 1000 kg of water. The ATTA-2 instrument had a limited dynamic range: it could only be used to count the rare 81′85Kr isotopes, not the abundant stable isotopes, for example, 83Kr whose isotopic abundance is 11.5%. The isotopic abundance 81Kr/Kr had to be measured in two steps: first, a controlled amount of 85Kr was introduced into the sample and its 85Kr/Kr ratio was determined with LLC; second, ATTA-2 was used to measure 81Kr/85Kr. The two ratios were then combined to obtain 81Kr/Kr. Despite its dependence on additional measurements with other techniques and the relatively large sample size required, ATTA-2 was used successfully for 81Kr-dating of old groundwater of the Nubian aquifer in western Egypt. However, none of these prior art methods and systems, including the existing ATTA-2 systems have enabled efficient or commercially practical methods and systems for detection and analysis of 81Kr/Kr or 85Kr/Kr ratios. Consequently, there is a substantial need for a commercially efficient method for Kr isotope measurements and analysis.